1. Field of the Invention
The present invention relates generally to an optical recording medium, on or from which information signals can be recorded or reproduced by an irradiation of beams, and to a method of manufacturing the same.
2. Related Background Art
Conventionally, it has been known that a thin film formed of a chalcogen material can be changed in phase between an amorphous phase and a crystal phase by being irradiated with laser beams. Utilizing this phenomenon, phase-change type optical recording media have been developed.
In such phase-change type optical recording media, it is required to facilitate the phase change of a phase change layer (a recording layer). Therefore, conventional optical recording media include a phase change layer and a layer laminated on the phase change layer and made of a material promoting crystallization of the phase change layer (see JP 5-342629 A, JP 9-161316 A, JP 11-73692 A, and WO 98/47142 A).
However, conventional optical recording media had the following problems. In a conventional optical recording medium, either mark portions or space portions are amorphous and usually the mark portions are amorphous. In general, the amorphous state includes some metastable energy states. When the amorphous material is stored for a long period or at a high temperature, an energy state of the amorphous material after storage may vary from the one before storage in some cases. Therefore, optimum recording and reproducing conditions differ between before and after the storage. Consequently, even if recording and reproduction are carried out under the same conditions, recording and reproducing characteristics may vary between before and after the storage. For instance, when the amorphous portions in a recording layer have been changed into a more stable energy state, the crystallization of the recording layer tends to be difficult. As a result, the erase ratio in overwriting of information signals decreases in some cases.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an optical recording medium having stable recording and reproducing characteristics even after a long term storage and to provide a method of manufacturing the same.
In order to achieve the above-mentioned object, an optical recording medium of the present invention includes a substrate and a recording layer positioned above the substrate, wherein the recording layer includes a phase change layer changing in phase reversibly between a crystalline state and an amorphous state by an irradiation of an optical beam and a crystalline nucleation layer positioned adjacent to the phase change layer for facilitating crystallization of the phase change layer. The phase change layer is obtained by being deposited in an amorphous state and then crystallized, and the crystalline nucleation layer contains Te in a range between 33 atom % and 67 atom %. According to the above-mentioned optical recording medium, an optical recording medium having stable recording and reproducing characteristics even after a long term storage can be obtained. This optical recording medium is based on a new finding obtained through experiments conducted by the present inventors, i.e. a new finding that stable recording and reproducing characteristics after a long term storage can be obtained by the use of a crystalline nucleation layer containing Te in a range between 33 atom % and 67 atom %. In this connection, the term xe2x80x9clayerxe2x80x9d used in the present specification includes a layer formed in an insular form.
In the optical recording medium, the crystalline nucleation layer may include at least one selected from a group consisting of Snxe2x80x94Te and Pbxe2x80x94Te. According to the above-mentioned configuration, an optical recording medium with particularly stable recording and reproducing characteristics even after long term storage can be obtained. This optical recording medium is based on a new finding obtained through experiments conducted by the present inventors, i.e. a new finding that the recording and reproducing characteristics after a long term storage are stabilized particularly through the use of the crystalline nucleation layer containing at least one selected from the group consisting of Snxe2x80x94Te and Pbxe2x80x94Te. In this connection, the above-mentioned xe2x80x9cSnxe2x80x94Texe2x80x9d has no limitation in the ratio between Sn and Te and may have any composition ratios within the above-mentioned range of Te content. Similarly, the above-mentioned xe2x80x9cPbxe2x80x94Texe2x80x9d also may have any composition ratios within the above-mentioned range of Te content.
In the optical recording medium, the phase change layer may contain Ge, Sb, and Te as constituent elements, and the atomic ratio among Ge, Sb, and Te in the phase change layer may be expressed as Ge:Sb: Te=X:Y:Z, where X+Y+Z=100, 10xe2x89xa6Xxe2x89xa645, 5xe2x89xa6Yxe2x89xa640, and 40xe2x89xa6Zxe2x89xa660. The aforementioned configuration enables an optical recording medium with a particularly high C/N ratio and erase ratio to be obtained.
The optical recording medium further may include first and second information layers positioned on or above the substrate and a separating layer positioned between the first and second information layers. At least one layer selected from a group consisting of the first and second information layers may include the recording layer. According to the above-mentioned configuration, an optical recording medium with a high recording density can be obtained.
In the optical recording medium, the crystalline nucleation layer may include at least one element selected from the group consisting of oxygen and nitrogen.
In the optical recording medium, the crystalline nucleation layer may have an extinction coefficient in a range between 0.5 and 2.0. According to the above-mentioned configuration, an optical recording medium with well balanced characteristics can be obtained.
In the optical recording medium, the phase change layer may have an average thickness between 4 nm and 14 nm. When the average thickness is set to be at least 4 nm, a tendency for the phase change layer not to be crystallized easily can be prevented. In addition, when the average thickness is set to be 14 nm or less, a high erase ratio can be obtained stably even after a long term storage.
Furthermore, in order to achieve the above-mentioned object, a method of manufacturing an optical recording medium according to the present invention is directed to a method of manufacturing an optical recording medium provided with a recording layer including a phase change layer and a crystalline nucleation layer positioned adjacent to the phase change layer. The method includes (a) depositing the phase change layer to be in an amorphous state, (b) forming the crystalline nucleation layer before or after the process (a), and crystallizing the phase change layer by an irradiation of an optical beam on the phase change layer in the amorphous state after the processes (a) and (b). The phase change layer changes in phase reversibly between a crystalline state and an amorphous state by an irradiation of an optical beam. The crystalline nucleation layer is a layer for facilitating the crystallization of the phase change layer and contains Te in a range between 33 atom % and 67 atom %. According to the above-mentioned manufacturing method, an optical recording medium having stable recording and reproducing characteristics even after a long term storage can be manufactured.
In the above-mentioned manufacturing method, the crystalline nucleation layer may contain at least one selected from a group consisting of Snxe2x80x94Te and Pbxe2x80x94Te.
In the above-mentioned manufacturing method, the phase change layer may contain Ge, Sb, and Te as constituent elements and the atomic ratio among Ge, Sb, and Te in the phase change layer may be expressed as Ge:Sb:Te=X:Y:Z, where X+Y+Z=100, 10xe2x89xa6Xxe2x89xa645, 5xe2x89xa6Yxe2x89xa640, and 40xe2x89xa6Zxe2x89xa660.
In the manufacturing method, the process (b) may include forming the crystalline nucleation layer in an atmosphere containing at least one element selected from a group consisting of oxygen and nitrogen, and the process (c) may include allowing the element selected from the group consisting of oxygen and nitrogen to leave from the crystalline nucleation layer. The above-mentioned configuration allows the crystalline nucleation layer to be formed easily in an amorphous state, thus facilitating the formation of the phase change layer in an amorphous state.